1. Field of the Invention
The present invention relates to an electric device in which a control microcomputer controls the electric device to stop its operation based on detection of occurrence of power stoppage in the power supply circuit.
2. Description of the Related Art
An electric device normally includes a power supply circuit which obtains an operating power to allow the electric device to operate by a power supplied from a commercial power supply. In addition, the electric device often includes a control microcomputer which controls an operation of the electric device.
In the electric device that includes the control microcomputer, the control microcomputer often detects an output of the power supply circuit through a power stoppage detection circuit. In the electric device of this type, if the power supply circuit does not output the operating power due to power stoppage or the like, the control microcomputer detects that the power supply circuit is stopped based on a signal input from the power stoppage detection circuit.
The power supply circuit often includes a power semiconductor switching element such as a power FET and obtains the operating power using an oscillation operation of this power semiconductor switching element.
If the electric device, which includes the power supply circuit and the control microcomputer as stated above, is in a stand-by mode in which the electric device does not operate and in which the start of the operation of the electric device is awaited, the output of the power supply circuit is normally suppressed to hold down unnecessary power consumption. The output of the power supply circuit in the stand-by mode is suppressed often by an intermittent oscillation operation of the power semiconductor switching element.
In order for the control microcomputer to detect a state of the operation of the electric device even if the electric device is in the stand-by mode, it is necessary to supply a voltage signal at a constant level to the control microcomputer from the power stoppage detection circuit.
In order to supply the voltage signal at the constant level to the control microcomputer even if the electric device is in the stand-by mode, the power stoppage detection circuit includes an assurance circuit comprised of a capacitor or the like that assures a signal level.
However, if the power supply circuit performs the intermittent oscillation operation, even the assurance circuit cannot often keep the signal input to the control microcomputer from the power stoppage detection circuit at the constant level.
If the level of the signal input from the power stoppage detection circuit to the control microcomputer does not reach the constant level, the control microcomputer erroneously determines that the power supply is stopped by power stoppage or the like.
If so, the control microcomputer makes the microcomputer itself stopped and controls the electric device to be stopped although the electric device is in the stand-by mode. As a result, even if the electric device is actuated to start its operation, the electric device cannot disadvantageously operate.